the contrary of what Blackwall assigned, was the reason adherence failed. Adherence likewise failed when the opposite side of the glass was moistened with ether, in consequence of the condensation of vapor occasioned by the evaporation of the ether.
Adherence also fails completely when the glass is covered with a thin wash of oil. And a fly which has been put upon a glass so covered, and is then transferred to a clean glass, will not be able to adhere to that till after some interval. An extremely thin coating of oil is enough to bring about a failure to adhere; even the rubbing of the finger on the glass is sufficient. The failure in this case is caused by the running together of the little drops of liquid on the hairs, by which the adhering surface is much reduced below the total surface presented by the little drops acting separately. Each foot then acts as a single hair, the diameter of which is equivalent to its own; and, even if its diameter were equivalent to a millimetre, the six feet bearing together upon the glass would not be competent to sustain the fly. For, according to the experiment with the horse-hair, a diameter
Fig. 3.—Foot of Polydrusus sericeus.—1. Pulvilli, with hairs and hooks. 2. Three hairs, considerably magnified. 3. A hair more considerably magnified.
of 0·12 of a millimetre will bear 0·00085 of a gramme; consequently, a diameter of a millimetre will bear 0·007 of a gramme, and the six feet together 0·042 of a gramme.
It is very difficult, if not impossible, for a fly to walk on a vertical polished surface when it is thinly covered with dust. When, after it
